"Given the observed level of activity noted, of a new tent, trucks, people and numerous portable fuel/oxidizer tanks, should North Korea desire - it could possibly conduct its fifth satellite launch event during the next three weeks," it said.

DigitalGlobe said the type of activity was consistent with preparations observed before North Korea's failed launch of its Unha-3 missile in April.

Pyongyang insisted the April launch bid was aimed at putting a satellite in orbit, but the United States and United Nations denounced the mission as a disguised ballistic missile test.

The test put a halt to international efforts to engage the isolated nation, with the United States calling off plans to deliver badly needed food assistance.

Any test in the next three weeks would cast a heavy cloud over South Korea's presidential election on December 19.

There have been widespread concerns in Seoul that the North would seek to influence the ballot by conducting a missile launch or provoking a border clash.

The Japanese Asahi Shimbun newspaper had reported last week that the US government had already warned Japan and South Korea that an imminent test was possible.

Last month, the US-Korea Institute at Johns Hopkins University said satellite images showed North Korea had conducted motor tests at the Sohae site to improve its long-range missiles.

Some analysts believe that a North Korean rocket, if successfully developed, could eventually reach the range to hit the United States.

North Korea is known to have an inter-continental ballistic missile (ICBM) in development - the Taepodong-2 - but it has never been tested successfully.

Days after the failed April test, North Korea raised eyebrows by displaying what appeared to be a new set of ICBMs at a military parade to mark the 100th birthday of the North's late founder Kim Il-Sung.

But Western military analysts and UN sanctions experts concluded that the display models were simply mock-ups.